The original "piston type" AR was the AR18/AR180 (the latter is the semi-auto version). It might well have been a better rifle than the AR15/M16, but the Army had already adopted the M16 and the AR18 never was fully developed.

It certainly would have been less expensive and easier to manufacture than the M16, being made from steel stampings, and is far less affected by dirt or sand than the M16.